Timing recovery is essential for any digital communications system employing coherent detection. In many systems (e.g., wireless), 30% of the transmitted energy is devoted to pilot tones that, among other functions, aid the timing recovery. In magnetic recording systems, similar strategies could be employed, but the price would be unacceptable as the disk area devoted to timing tones would lower the area available for the storage of data.
A typical disc drive includes one or more discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the discs and to store information on the discs.
In one conventional disc drive, an electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations.
Information is typically stored in concentric tracks on the surface of the discs by providing a write signal to the data head to write information on the surface of the disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the disc, sensing the information on the disc, and generating a read signal based on the information. The read signal is typically conditioned and then decoded by the drive read/write channel and the controller to recover the data.
A Viterbi detector has been used in the past as a data detector in a disc drive read channel. A Viterbi detector acts as a maximum-likelihood sequence estimator when the input to the detector consists of a signal plus additive white, Gaussian noise, and when a typical branch metric (e.g., the square of the error in the signal provided to the detector) is used.
The magnetic recording channel is a digital baseband communication channel with intersymbol interference, non-stationary noise and channel memory. Recent results have shown that Gauss-Markov noise approximations lead to nearly optimal signal detection methods.
As recording densities of disc drives increase, there is an increasing probability of interference between adjacent areas of magnetization in the storage media. Signals detected by magnetic read heads are typically sampled at times controlled by a clock signal. Variations in the timing of clock pulses can produce timing error in the samples. Thus there is a need for a method and apparatus for symbol recovery that can be incorporated into a receiver that receives data symbols which are subject to intersymbol interference and timing error.